The invention relates to a luminaire comprising:
a reflector body having a reflective part provided with a coating, the coating in which exists at least a first and a second interference layer, the layers being mutually different, the coating further comprising at least one material selected from a set consisting of materials with a high-index of refraction and at least one further material selected from a further set consisting of materials with a low-index of refraction;
contact means for electrically connecting a light source.
Such a luminaire is known from U.S. Pat. No. 3,644,730. In the known luminaire the coating is light reflecting and comprises two or more interference layers of one-quarter wavelength each, the layers are alternatively of high- and low-index material. By choosing the number of layers, their index of refraction and their respective thickness the coating can be given particular desired optical properties. The optical properties of the coating are based on interference of light, the material of the interference layers being partly transparent for light. The interference is used to selectively influence wavelength dependence of reflection and transmission of the coating. It is thus enabled for the coating to be selectively reflective, for example, to be transparent for IR-radiation whilst being reflective for visible radiation. It is a disadvantage that the manufacturing of the reflective coating is cumbersome since for the coating to appear white, i.e. the coating being essentially total reflective for all wavelengths of the visual spectrum, a large number of alternate layers of high- and low-index materials is required. The manufacturing is even more cumbersome as it is difficult to apply the coating on the curved/shaped surface of the reflecting part of the luminaire. Alternatively, when a coating step is done before a shaping step, the manufacturing is even so cumbersome as the shaping of the pre-coated reflector involves significant risk of damage to the coating.
In a backlighting system the light reflecting coating might simultaneously act as a coating for a diffusor, i.e. due to scattering by the coating light passing through the diffusor is diffused. For example titanium dioxide particle coatings are generally known for that purpose. For such scattering of light to occur effectively, the coating should comprise particles having a size in the order of the wavelengths to be scattered, i.e. in the range of less than 1 xcexcm. However, conventional coatings of essentially white particles of the indicated size, for example generally known titanium dioxide, suffer from color shift due to wavelength dependent scattering.
It is an object of the invention to provide a luminaire of the kind as described in the opening paragraph in which the abovementioned disadvantages are counteracted.
In accordance with the invention the luminaire of the type as described in the opening paragraph is characterized in that the coating comprises at least a first and a second light reflective particle group, the first interference layer being provided on particles of the first particle group, and the second interference layer being provided on particles of the second particle group,
for each light reflective particle group, the particles of that particle group consist of a material selected from one of said sets, and the respective interference layer consists of a material selected from the other of said sets,
at least one material in each respective light reflective particle group is selected to be different in composition or layer thickness from materials of any other particle group, and
relative quantities of each respective light reflective particle group are chosen such that, when their reflections are blended, white light of predetermined CIE coordinates is produced.
A generally known method for obtaining white light by blending relative spectral proportions is described in Van Nostrand""s Scientific Encyclopedia by Douglas M. Considine, Van Nostrand Reinhold Company, New York (1976), 5th edition. In U.S. Pat. No. 4,434,010 a method to manufacture particles with an interference layer is disclosed. Particles with an interference layer are commercially available, for example under the trade name Iriodin/Afflair, and exhibit pearlescence, i.e. the particles have a milky brightness. The color of the pearlescent particles is due to the interference of light, i.e. interference of a part of the visible spectrum. In comparison thereto, conventional pigments absorb a part of the visible spectrum, while luminescent materials emit a part of the visible spectrum. The mutually different color of the interference (or pearlescent) pigments and thus of the particle groups is due to the interference layers being mutually different. For example, the interference layers of the first particle group with respect to the second one can differ either in layer thickness or in index of refraction, for example in that they are made of different materials. The particles preferably have a relatively large size, i.e.  greater than =5 xcexcm, and for that reason wavelength dependent scattering and hence color shift is counteracted. In the coating applied in the inventive luminaire, the particles in general have a relatively random orientation compared to the orientation of a layer on the shaped surface of the reflecting part of the known luminaire. It is generally known that the reflectance and color appearance of an interference layer is dependent on the wavelength and the angle of incidence of light. However, it was observed that the coating in the luminaire of the invention exhibits less dependency both on the incident angle of light and on the view angle on the coating. This can be explained by the relatively random orientation of the particles, and thus of the interference layer provided thereon, or the use of the blend of the different particle groups wherein a coloring effect of one particle group is more or less compensated for by another particle group. Preferably each particle within one of the particle groups is provided with the respective interference layer, and so further improving the independency of incident angle of light and view angle with respect to the color appearance. When the coating comprises at least two groups of mutual differently colored particle groups in appropriate relative proportions, it is possible to effectively counteract that the coating exhibits a particular color. Surprisingly, it appeared that the colors of the particle groups don""t behave as subtractive colors as is the case for pigments, i.e. the combination of colors leads to darker/black colors. On the contrary, the colors of the particle groups behave as additive colors as is the case for luminescent materials, i.e. the combination of colors lead to whiter colors. Thus a coating which appears white for the human eye is obtainable. Such a white coating is especially well obtainable when in the coating the particles of said particle groups are mixed instead of being stacked as separate layers on each other. Coatings consisting of particles are relatively easily applied, for example by spraying, onto the reflector body, thus enabling the relatively easy manufacture of the luminaire having a white coating. It appeared that the coating of pearlescent particles has a relatively high reflection and that the interference layer is practically fully transparent for light. As a result, said coating has the advantage that larger numbers of reflections inside the coating and/or variations in thickness in the coating do not lead to significant light loss or to a color shift. Such light loss and/or said color shift, however, can be observed by conventional white powder coatings with optimized scattering power, such as, for example, a coating comprising titanium dioxide particles.
When a combination of two particle groups is used in the coating, the choice of the first particle group is determined in relation with the second particle group. The particle groups each have respective color coordinates in the CIE x,y-chromaticity diagram. A line drawn in the CIE x,y-chromaticity diagram between the color coordinates of the respective particle groups crosses an area of color coordinates of light which has a white appearance to the human eye, i.e. the white color area. In said coating the relative quantities of the two particle groups are chosen in proportion to the length of the section of the drawn line from the respective color coordinates to the white color area, so that when their reflections are blended, there is produced light with color coordinates of light that has a white appearance to the human eye. A generally known method for obtaining white light by blending relative spectral proportions is described in Van Nostrand""s Scientific Encyclopedia by Douglas M. Considine, Van Nostrand Reinhold Company, New York (1976), 5th edition. A favorable combination of two particle groups is, for example, a blue colored particle group with a gold colored particle group, as in this case in the white color area the drawn line between the color coordinates of the respective particle groups in this case runs substantially parallel to the black body line. This offers the advantage that the relative quantities of the two particle groups can be varied over a relatively wide range whilst a white color appearance is still obtainable. This combination of two groups enables in a relatively simple way the manufacture of a coating which appears white for the human eye. When three particle groups are used in the coating, the particle groups are chosen such, that the triangle formed by the color coordinates of the particle groups in the CIE x,y-chromaticity diagram encloses the white color area. The same reasoning goes for a coating comprising four or more particle groups. A favorable combination of three particle groups is, for example, a blue colored particle group with a green colored particle group and a red colored particle group. The combination of these three groups enables relatively easy to obtain a coating with a specific white color impression and/or makes an even wider range of coatings with a different white color obtainable than is obtainable with two particle groups.
In another embodiment of the luminaire according to the invention the coating comprises four or five particle groups, the coating is particularly suitable for luminaires in which a relatively large number of reflections of light occur, for example in a backlighting system. In such backlighting systems often a diffusor is provided with a coating purposely having a variation in thickness to diffuse the light originating from the light source, which light subsequently is used to homogeneously illuminate a screen. In the event that the reflection of the coating is dependent on the wavelength of the visible spectrum, each reflection results in a color shift, as one part of the spectrum is reflected more efficiently than another part of the spectrum. When only a small number of reflections are involved, said color shift often is not distinguishable by an observer. However, when a relatively large number of reflections are involved, as is often the case in a backlighting system, the color shift is enhanced and might become visible. The visibility of the color shift is enhanced when areas of the diffusor with color shift and areas without color shift are adjacent (or border) each other. By the number of groups in the coating being four or five, it is possible to give the coating a reflection that is practically constant for the visible range of the spectrum, enabling color shift due to thickness variation of the coating to be reduced to an acceptable low level. Furthermore the number of groups being four or five in the coating, renders the coating to be less sensible for local inhomogeneities which otherwise may lead to color differences exhibited by the coating. Moreover it is easier to obtain the white appearance of the coating as the white appearance of the coating is less sensible to fluctuations in composition of the coating and layer thickness variations of the interference layer on the particles. From experiments it was apparent that the respective particle groups exhibiting respectively a blue, green, red, gold and platinum-gold color, are in particular suitable to obtain the desired, homogeneous white appearance of the coating.
The luminaire according to the invention is in particular suitable as a back-lighting system, for example in a liquid crystal display (LCD) system. In back-lighting systems a large number of multiple reflections are required to obtain a homogeneous distribution of light which light subsequently is to be supplied to the LCD. In conventional systems said large number of multiple reflections leads to effects of relatively large light losses and/or to color shifts, said effects being counteracted by the luminaire according to the invention comprising said interference coating.